The effect of flywheel training on
functional neuromuscular performance
in physically active youth
Niklas Westblad
GYMNASTIK- OCH IDROTTSHÖGSKOLAN
Swedish School of Sport and Health Science
Master thesis 47:2018
Master program 2016-2018
Supervisor: Glenn Björklund
Vice supervisor: Niklas Psilander
Examiner: Victoria Blom
Abstract
Aim
The aim of this study was to investigate the effect of flywheel resistance training on functional neuromuscular performance in physically active youth.
Method
Forty-four healthy and physically active youth between 12-14 years of age (n=19 boys & n=25 girls) volunteered to participate and were randomized into three different groups of flywheel resistance training (FRT) (n=15, body mass = 42,9 ± 8,6 kg, time to Peak Height Velocity (PHV) = - 0,8 ± 1,6), traditional strength training (TST) (n=15, body mass = 44,7 ± 10,3 kg, time to PHV = - 0,8 ± 1,5) and a control group (CON) (n=14, body mass = 43,8 ± 9,0 kg, time to PHV- 0,8 ± 1,5. Squat jump (SQ), Countermovement jump (CMJ), 10-m
acceleration, 20-m speed and 30-sprint was assessed pre- and post-intervention. All training groups performed 12 resistance training sessions over a 6-week intervention. The FRT-group performed bilateral flywheel resistance squats with 4 sets of 6 repetitions with 0,025 to 0,05 kgm2 and the TST-group performed bilateral barbell squats with 4 sets of 6 repetitions at a predicted 80 %1RM, while the control group only performed their regular sports training.
Results
Repeated measures two way-ANOVA, 3 x 2 (training group x time), showed no significant mean effects between groups after the intervention. A significant increase occurred in the control group for SQ; 2,4 ± 2,5 (cm) p ≤ 0,008 and CMJ; 2,2 ± 3,1 (cm) p ≤ 0,037. Both training groups increased significantly in body mass from pre- to post-tests by 2,0 ± 2,7 kg for the flywheel training group and 1,3 ± 0,9 kg in the traditional strength training group (p ≤ 0,05).
Conclusions
This study indicates that flywheel training can be used as a resistance training method for youth athletes without inducing training related injuries. Flywheel resistance training resulted in a small but non-significant increase from pre to post test in squat jump and 10-m sprint. Future studies on flywheel resistance training for youth needs to investigate the
implementation of longer training periods, additional training sessions, more experienced youth in resistance training and faster movement speed.
Sammanfattning
Bakgrund
Syftet med denna studie var att undersöka effekten av svänghjulsträning på funktionella neuromuskulära tester hos fysiskt aktiva barn.
Metod
Fyrtiofyra friska och fysiskt aktiva barn i åldrarna 12–14 (n=19 killar & n=25 flickor) deltog frivilligt och randomiserades in i tre olika grupper med svänghjulsträning (n=15, kroppsvikt = 42,9 ± 8,6 kg, tid till Peak Height Velocity (PHV) = - 0,8 ± 1,6), traditionell styrketräning (n=15, kroppsvikt = 44,7 ± 10,3 kg, tid till PHV = - 0,8 ± 1,5) och kontrollgrupp (n=14, kroppsvikt = 43,8 ± 9,0 kg, tid till PHV- 0,8 ± 1,5). Squat jump (SQ), Countermovement jump (CMJ), 10-m acceleration, 20-m hastighet and 30-sprint mättes före och efter interventionen. Båda träningsgrupperna genomförde 12 styrketräningspass över en
6-veckorsperiod. Svänghjulsgruppen genomförde bilaterala knäböj med 4 set och 6 repetitioner mellan 0,025 till 0,05 kgm2 och den traditionella styrketräningsgruppen genomförde bilaterala knäböj med skivstång bak om 4 set med 6 repetitioner på en uppskattad belastning av
80%1RM. Kontrollgruppen genomförde endast sin dagliga idrottsträning.
Resultat
En upprepad tvåvägs-ANOVA, 3 x 2 (träningsgrupp x tid), visade ingen signifikant effekt mellan grupperna efter denna intervention. Kontrollgruppen var den enda som ökade i SQ; 2,4 ± 2,5 (cm) p ≤ 0,008 och CMJ; 2,2 ± 3,1 (cm) p ≤ 0,037. Båda träningsgrupperna ökade i kroppsvikt från pre- till post-test med 2,0 ± 2,7 kg för svänghjulsgruppen och 1,3 ± 0,9 kg för den traditionella styrketräningsgruppen (p ≤ 0,05)
Slutsats
Den här studien indikerar att svänghjulsträning är ett potentiellt belastningsalternativ för styrketräning av ungdomar utan att resultera i träningsrelaterade skador. Svänghjulsträning resulterade i en liten men icke signifikant ökning från för- till eftertest i Squat jump och 10-m sprint. Framtida studier på ungdomar och svänghjulsträning bör undersöka effekten av längre träningsperioder, fler träningspass, effekten av mer erfarna ungdomar och ökad
Table of Contents
1 Introduction ... 1
1.1 Aim and research question ... 5
2 Method ... 6 2.1 Subjects ... 6 2.2 Protocol overview ... 6 2.2.1 Anthropometrics ... 7 2.2.2 Familiarization ... 7 2.2.1 Jump tests ... 7 2.2.2 Sprint testing ... 8 2.2.4 Training intervention ... 8
2.4 Validity, reliability and ethics ... 9
2.5 Statistical analysis ... 10
3 Results ... 11
3.1 Participants ... 11
3.2 Neuromuscular performance tests ... 11
3.2.1 Vertical jumping tests ... 14
3.2.2 Sprinting tests ... 14
4 Discussion ... 14
5 Conclusion ... 19
Appendix 1 Source and Literature search
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1 Introduction
Scientific evidence supports that regularly participating in well programmed and supervised resistance training strengthens health-, fitness- and sports performance adaptions for youth (Bergeron et al. 2015; Faigenbaum & Myer 2010; Lloyd et al. 2014a; Zwolski, Quatman-Yates & Paterno 2017). Despite theories that physical activity and specifically resistance training leads to physical harm of secondary sex characteristics such as, delayed menarche and restriction of growth height, researchers in the pediatric field has dispelled these myths in position statements from the NSCA (Lloyd et al. 2016a) and the International Olympic Committee (Bergeron et al. 2015). On the contrary, youth athletes that regularly participate in appropriately designed resistance training improves their bone mineral density and skeletal health (Álvarez-San Emeterio et al. 2011) and reduces their risk for sports-related injuries (Faigenbaum & Myer 2010; Lloyd et al. 2014a).
In the position statement on Long-Term Athletic Development, by the National Strength and Conditioning Association (NSCA), authors state that “All youth should be encouraged to enhance physical fitness from early childhood, with a primary focus on motor skill and muscular strength development” (Lloyd et al. 2016a). Youth athletes are defined as both children up to approximately 11 years of age for girls and 13 years for boys, and adolescents, girls aged 12-18 and boys 14-18 (Lloyd et al. 2016a). Motor skills consists of running, jumping and throwing (Behringer et al. 2011) and muscular strength is generally defined as the ability of muscle or muscle group to generate maximal force at a specified velocity (Smith et al. 2014). Resistance training that is well-supervised has the potential to improve muscle strength (Behringer et al. 2010) and motor skills (Behringer et al. 2011; Lesinski, Prieske & Granacher 2016) in youth.
Besides inducing positive health effects, implementing resistance training in early stages of childhood would increase the time to improve technical and physical abilities, in a progressive manner. Abilities that could help the individual to tackle the demands of either a life as an athlete or as a recreational exerciser. Adult athletes that possess great maximal strength tends to produce greater rate of force development, produce larger mechanical power output, jump higher, perform better in change of direction situations and are more resilient towards injury risk in comparison with athletes with lower maximal strength ability (Suchomel, Nimphius &
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Stone 2016). Youth athletes that implement resistance training in their daily routine will therefore benefit from the increased ability to tolerate the demands of long-term training and competition (Granacher et al. 2016).
Youth of the same chronological age can differ up to as much as four to five years in their biological maturation during puberty (Lloyd et al. 2014a; Lloyd et al. 2014b). Knowing the predicted status in biological maturation of a youth is important, since youth pre-puberty seems to increase their muscular strength due to neuromuscular adaptations with no change in muscle architecture (Blimkie 1993). The current recommendation of The American College of Sports Medicine (ACSM) is that a child is ready to participate in carefully prescribed and supervised resistance training when they are mentally mature to participate in a group setting and follow instructions (Myer et al. 2014). That is, in terms of chronological age, around the age of 6 to 7, which is the same age as when children often start to participate in regular sports activities, such as soccer, gymnastics, ice hockey.
The growth period when the height of a youth peaks is referred as the peak height velocity (PHV) and is frequently used in the literature to separate children pre- and mid-puberty (Beunen & Malina 1988). The peak height velocity generally occurs in girls around the age of 12 and in boys around the age of 14 (Lloyd & Oliver 2014, s. 14). Youth individuals, mid- or late-puberty will continue to develop their neuromuscular ability as well as structural and architectural changes will occur in the muscle due to increased hormonal concentrations (Lloyd et al. 2014a; Malina, Bouchard, & Bar-Or 2004). Therefore, coaches should be aware of this when planning a strength session for individuals during the time of puberty (Lloyd et al. 2014a; Lloyd et al. 2014b). Progressively increasing the exercise intensity through external load with gravity dependent free weights as barbells, dumbbells and resistance bands, have been shown to induce greater improvements in strength more than growth due to natural maturation (Lloyd et al. 2014a).
The general recommendations for youth resistance training is a training frequency of 2-3 sessions per week, 2-4 sets with 6-12 repetitions and an intensity of 50-80 % of 1RM (Lloyd et al. 2014a). Specific recommendations to increase the muscular strength of youths (6-18 years old) was presented by Lesinski et al. (2016) in their meta-analysis of the relationship between effects and dose of resistance training on physical performance in youths. The
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recommended outcome of their analysis was that a training period of 23 weeks, 5 sets per exercise, 6-8 repetitions per set, a training intensity of 80-89 % of 1-repetition maximum (1RM) and 3-4 min rest between sets were most effective to increase maximal strength in youth. However, 23 weeks is a long time for practically applying resistance training in youth and it could be more practically applicable to implement a shorter block of resistance training in youth. Therefore, the 6-week period that has been used in earlier studies and been a recommended as mesocycle in youth programming could be more appropriate (Lloyd et al. 2016b; Haff 2014, p. 162). Programming recommendations aside, most important is that the individual can demonstrate sound technical proficiency. Then, the external load needs to be applied in a progressive manner to induce performance enhancements beyond the natural effects of maturation and growth (Lloyd et al. 2014a).
The effects of resistance training can be measured in several ways and the most common method is to assess a person’s 1RM before and after a training period. Even though studies indicate that the traditional 1RM-testing is safe for both children (Faigenbaum, Milliken & Westcott 2003) and adolescents (Faigenbaum et al. 2012), assessing their neuromuscular performance through functional movements as sprint- and jump tests, might be more time efficient than learning a barbell squat. In a recent study by Lloyd and colleagues (2016b) they studied the effects of different resistance training methods by assessing the functional
neuromuscular performance of young male school children pre- and post-PHV. All training groups improved in measures of sprint and vertical jumping irrespective of the resistance training mode or maturity (Lloyd et al. 2016b). Assessing the neuromuscular performance might therefore be more applicable in a large group setting. It might also be an assessment method with good buy-in from the youth since most youth athletes regularly sprint and jump in their daily sports training.
Traditional strength training, plyometric training and combined strength and plyometric training has all been proven to increase the neuromuscular performance in youth athletes (Behringer et al. 2011). An alternative resistance training method, that to the best of the author’s knowledge hasn’t been examined on youth pre- and mid-PHV is flywheel resistance training. Flywheel resistance training relies on resistance created by inertia, instead of the traditional gravity dependent strength training. The intensity of flywheel resistance training
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depends both on the number of flywheels and its size, as well as the effort put into the movement by the exercising individual.
Figure 1. Flywheel squat.
To improve an ability, such as the maximal strength in the lower body, the first step is to decide what exercise to perform to improve that specific ability. The second phase would be to achieve the ability to perform the movement in a proficient way. And the last step would be to increase the load of the exercise through either external load or an increase in total training volume. The proposed model of function, technique and exercise loading is conceptualized in figure 2. The harness that individuals wears when performing a flywheel squat attaches to the strap just in front of the individual’s pelvic bone (figure 1). The flywheel squat might provide a biomechanical advantage as the load is applied on the pelvic bone and the legs through the harness in comparison with a traditional barbell squat where the load is applied on the upper back. It might also be easier to learn the technical movement when performing a bilateral squat with the harness. In an applied setting, this could lead to a faster process through the technical learning phase and enable the possibility to faster load the youth athlete, still in a safe manner.
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Figure 2. A conceptual model of a progressive manner for choosing an exercise with the desired function, achieving technical competency of the movement and then being able to apply more load to the exercise.
There are currently no available recommendations for youths when performing flywheel training, due to an insufficient research of the area. Flywheel training has yet to be studied on a larger scale when it comes to young athletes and hasn’t been studied on youth athletes younger than the age of 18 (De Hoyo et al. 2015). However, it has been shown that flywheel training can induce muscular strength gains as good as traditional strength training in adults (Vicens-Bordas et al. 2017) and flywheel training has also been shown to induce robust increases in a sprint and vertical jumping performance (De Hoyo et al. 2015). A meta-analysis on flywheel training for adults indicates that a periodized program of 2-4 sets and 6
repetitions seems to induce the biggest effects on horizontal and vertical displacement
(Maroto-Izquierdo et al. 2017b). As for exercise intensity, a novel approach when introducing youth athletes to flywheel training would be to start with a small flywheel of 0,025 kgm2. Flywheel resistance training, with a study design of 6 weeks with 4 sets of 6-7 repetitions has produced performance enhancements in squat jump, countermovement jumps and sprint times in adult participants (Núñez et al. 2018; Sabido et al. 2017; Maroto-Izquierdo et al. 2017a; Naczk et al. 2016; De Hoyo et al. 2015; Fernandez-Gonzalo et al. 2014).
1.1 Aim and research question
The purpose of this study is to investigate the effect of flywheel training in comparison with traditional strength training on functional neuromuscular performance tests.
What is the effect of flywheel resistance training on functional neuromuscular
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2 Method
2.1 Subjects
Forty-four healthy and physically active youth between 12-14 years of age (n=19 boys and n=25 girls) from an athletic club in Stockholm volunteered to participate in the study. All subjects and parents were informed about the purpose of the study and all parents signed a written informed of consent prior to the pre-tests. Inclusion criterions was 12-14 years of chronological age, physically active at least two times a week at the athletic club and physically healthy.
Table 1. Physical characteristics of participants pre-intervention (mean ± SD).
Statistical analyses showed no significant differences between groups.
2.2 Protocol overview
All participants’ anthropometrics were assessed when accepted to the study (table 1). The participants performed three familiarization sessions consisting of 3 sets of 10 bilateral squats with a flywheel device (kBox 4 Pro, Exxentric, Sweden) and 3 sets of 10 bilateral squats with a 10 to 20-kilogram barbell. After completing three familiarization sessions all participants performed functional neuromuscular performance tests in the following order;
countermovement jump without arm-swing, squat jump, 30-m sprinting and maximal isometric force. Participants were then randomized into three different groups; a flywheel training group, a barbell training group and a control group that continued with their regular sports training. Training outside of this study was not standardized. Both training groups performed two resistance training sessions a week during a total of six intervention weeks. Participants trained two times a week for three weeks, then rested for one week and continued training two times a week for an additional three weeks. Both training groups performed 4 sets of 6 bilateral squat repetitions in their respective training mode. After the training period, the participant’s body mass was re-assessed, and all neuromuscular performance test was performed. Ten sessions were set as the least amount to complete to be included in the study.
Group Sample size Height (cm) Body mass (kg) PHV Gender
Flywheel RT 15 155.0 ± 7,6 42,9 ± 8,6 - 0,8 ± 1,6 10 girls, 5 boys
Traditional RT 15 156.8 ± 8,9 44,7 ± 10,3 - 0,8 ± 1,5 6 girls , 9 boys
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2.2.1 Anthropometrics
All participants standing height (centimeters) and seated height (centimeters) was measured using a stadiometer attached to a wall. Body mass (kilograms) was assessed using an electronic scale (FitScan BC-545F, Tanita Corporation of America). The average of three assessments was set as the subject’s body mass in kilograms. Anthropometric data was also used to predict if the subject was pre- or post-PHV (Mirwald et al. 2002). PHV was calculated with the equations proposed by Mirwald et al. (2002). The equations require the assessment of chronological age, body mass, standing height and seated height. There are two separate equations for male or female which can determine the PHV within a standard error of approximately 6 months (Mirwald et al. 2002). Time to peak height velocity was used for block-randomizing participants into their respective groups. Participants were divided into blocks of three starting from the oldest in predicted PHV to the youngest. A participant was then randomized into each group out of every block.
2.2.2 Familiarization
All participants performed three familiarization sessions to accustom themselves to the squat- and jump movement. Three sessions with flywheel resistance training has been used in earlier studies to let the participants acquaintance themselves with the flywheel movement and minimize effects of learning (Sabido, Hernández-Davó & Pereyra-Gerber 2018). Participants were instructed and then tried the vertical test-jumps to land correctly according to the testing standardization. Sprint familiarization was not considered necessary since participants
sprinted on a regular basis in their daily training. The resistance training familiarization consisted of 3 sets of 10 bilateral squats with a 0,025 kgm2 flywheel and 3 sets of 10 bilateral squats with a 10, 15 or 20-kilogram barbell. Participants were instructed to perform a full depth squat with the femur trochanter major lower than the lateral femur epicondyle. Individual feedback was given if a participant couldn’t perform a correct squat movement.
2.2.1 Jump tests
Countermovement jump height (centimeters) and squat jump height (centimeters) were recorded using a photocell system that calculates the jumping height of the time in the air (IVAR Jump and speed analyzer, LN Sport consult, Sweden). In the countermovement jump participants were instructed to perform a vertical jump from a squatting depth of their own choice with an encouragement to perform the eccentric phase as fast as possible to maximize jump height (Lloyd et al. 2009; Cormack et al. 2008). Squat jump was performed from a
half-8
squat position in a 90o knee angle (subjectively approximated by the supervising test official) from a 2-second isometric start before jumping vertically (Lloyd et al. 2009). For both tests, participants were instructed to hold their hands akimbo during the entire jump and land with a rebound jump to naturally land on their toes with a straight leg. Each participant performed three attempts for both Squat jump and Countermovement jump, their best attempt was chosen for further analysis.
2.2.2 Sprint testing
Sprint times were recorded using wireless timing gates (Brower TC Timing System, Draper, Utah, USA) with single-beamed photocells. Data were collected at 10 and 30 meters with a stationary start 0,3 (m) behind the first timing gate (Altmann et al. 2015). Each participant performed three attempts and the best attempt was chosen for further analysis.
2.2.4 Training intervention
Resistance training was performed twice a week for 6 weeks. There was a minimum 48 hours rest between each session to allow participants adequate recovery from respective resistance training session. Participants had the opportunity to choose between Monday, Wednesday or Friday for their two training sessions. All training sessions and every repetition was
supervised by fully qualified trainers. A correct technical execution was stressed during the entire training period and if a technical execution wasn’t satisfactory, a coach stopped the set to avoid a potential risk of injury to the participant. The intensity of the exercise was only increased if technical execution was satisfactory. If failing to perform a correct technical squat movement, the intensity was either decreased or the participant received individual technical feedback to perform the movement in a sound way.
Each session started with an aerobic 10-minute standardized warm-up performed either by running or on a bicycle ergometer. The barbell-training group did 1-2 warm-up sets with 50-70 % of the workload. The flywheel-training group only performed the aerobic warm-up. Training sessions lasted no longer than 30 minutes and inter-set rest periods was between 2-3 minutes. Both groups performed 4 sets of 6 repetitions with the only exception that the flywheel training group performed 2 pre-reps to initiate the rotational force of the flywheel. The flywheel training group started with a 0,025 kgm2 flywheel and the traditional strength training group started with an external load between 10 – 30 kg, depending on what the trainer decided was enough.
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To achieve the desired training intensity of 80% 1RM a self-reported rating of perceived exertion (RPE) method with reps in reserve (RIR) was used to adapt the training intensity (Campbell et al. 2017; Zourdos et al. 2016). The RPE-scale consisted of a 1 to 10 scale with 1 being easy and 10 hard. All participants were instructed with an experimental scale for RPE in resistance training. The RPE values corresponded to a RIR-value. After each set, the
participant was asked to self-report the RPE-value of the set and trainers adapted the applied load to achieve a set-RPE of 8 (8 RPE = 2 repetitions remaining). With an estimated 2-reps in reserve, it was believed to achieve enough training stimulus while maintaining acceptable movement performance.
Movement speed on the flywheel device was measured with a Bluetooth application (kMeter Exxentric, Sweden). When a participant performed a flywheel squat with movement
excellency and a movement speed in the range of 0,5 – 0,6 m/s, the supervising training increased the inertia from one 0,025 kgm2 flywheel to one 0,05 kgm2 flywheel. Chosen movement speed was adapted from Zourdus et al. (2016) to represent an estimated exercise intensity of 80 %1RM. No participant trained with a higher intensity than 0,05 kgm2. Exercise intensity for the traditional strength training group was increased to match a set-RPE 8 if technical performance was acceptable. Exercise intensity for the traditional strength training group ranged from 10-75 kg.
2.4 Validity, reliability and ethics
Haugen and Bucheit (2016) report a reliability of single-beamed photocells Standard Error of Measurement to 0,03 s and ~2% CV for single-beamed timing. They suggest that single-beam timing should be avoided if possible, but if used, the typical error can be decreased by
performing 4-9 repeated measures. Due to the limited amount of time, only three sprints were made. A comparison between single-beamed and dual-beamed timing systems shows absolute differences in time of -0,05 – 0,06 s in 0-20 m sprint (Haugen et al. 2014). Starting timing-gates was placed in a low height (0,6 m) to reduce the chance of a measurement error by preventing both the legs and the arms to break the photocell (Haugen & Bucheit 2016). The first timing-gate was placed 0,3 meters away from the starting line in accordance to Altmann et al. (2015) to standardize the starting distance.
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Photocell systems consists of two parallel bars with one receiver and one transmitter that indirectly measures the participants time in the air and converts it to jumping height
(Glatthorn et al. 2011). Photocell systems have been validated against the golden standard in force plates for squat jump and countermovement jump and has shown strong concurrent validity and excellent test-retest reliability (Glatthorn et al. 2011). IVAR jump system has been used in earlier studies when assessing Countermovement jump data (Parker et al. 2017)
Squat jump has been used to evaluate concentric strength of the leg extensors while countermovement jump has been used to measure the reactive strength component of the lower body with the inclusion of the eccentric phase (Cronin et al. 2004; Arteaga et al. 2000;
Wilson, Murphy, & Giorgi 1996). Both Squat jump and Countermovement jump have been shown to be the most valid and reliable field tests for estimation the explosive power of the lower limbs in physically active men (Markovic et al. 2004). Squat jump has also been shown to be a moderate reliable test when evaluated in youth athletes (Lloyd et al. 2009). To ensure test-retest validity and reliability a test official supervised the same test assessment for both pre- and post-test. All training was supervised by the same trainer in the respective training mode to ensure exercise intensity was adapted in the same way for all participants.
The Bluetooth application that was used for measuring movements speed has been shown to provide valid measurements but not so reliable measurements (Bollinger et al. 2018).
Movement speed was only used as an extra guiding tool for the supervising trainer to make an even more informed decision as when to increase flywheel inertia.
All participants and their parents were informed with the purpose and risks of participating in the study and all parents to the participating children signed a written informed of consent (Vetenskapsrådet u.å.) (see appendix 1). This study is approved by the Swedish regional board of ethics, DNR 2018/765-31/1.
2.5 Statistical analysis
All values are reported as means ± SD or percent change or both from pre and post the training intervention with a confidence interval of 95%. Statistical analyses were performed with IBM SPSS Statistics Version 22 (IBM, New York, USA). Data was analyzed for normal distribution by running a Shapiro Wilks test. The difference of all performance variables was
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analyzed using a 3 x 2 (training group x time) repeated measures two way-ANOVA, where “training group” represents flywheel training, traditional resistance training or control group, and “time” means pre- to post-training data. The sphericity of data was tested by Mauchly’s Test of Sphericity and was not violated. Difference from pre-training to post-training in groups was analyzed by performing a dependent t-test. An alpha level of 0,05 was set a priori. Partial Eta Squared was calculate and used as effect size (ES) to determine the
meaningfulness of the differences. Partial Eta Squared was interpreted as small Ƒ= 0,1, medium Ƒ= 0,25 and large Ƒ= 0,5 (Richardson 2011).
3 Results
3.1 Participants
Thirty-seven (n = 37) of forty-four (n = 44) participants completed the study. Two participants were sick at the time for post-testing but completed the entire training
intervention. Three participants had to quit prematurely due to sickness leading to missing too many sessions. Two participants in the control group didn’t show up for the post-tests. Only body mass was re-assessed after the training intervention (table 2). Both training groups received a significant increase in body mass by 2,0 ± 2,7 kg (p ≤ 0,014) for the flywheel training group and 1,3 ± 0,9 kg in the traditional strength training group (p ≤ 0,001). No significant change in body mass occurred for the control group. No musculoskeletal injuries were reported during the intervention.
3.2 Neuromuscular performance tests
The flywheel training group completed a mean of 96 % of the total training sessions and the traditional resistance training group performed a mean of 95 % of the total training sessions.
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3.2.1 Vertical jumping tests
Two way-ANOVA showed no differences between groups. There was a medium time effect on SQ: ES=0,32 F(2,38)=15.96, P<0,0005 and a small time effect on CMJ; ES=0,12,
F(2,38)=4.73, P<0,05. Only the control group gained a significance increase in mean jumping height in both jumps by, SQ; 2,4 ± 2,5 (cm) p ≤ 0,008 and CMJ; 2,2 ± 3,1 (cm) p ≤ 0,037. There was only a trend in change for the traditional strength training group in the squat jump by 1,9 ± 2,8 (cm) p ≤ 0,053. No significant differences were made from pre to post-tests in SQ and CMJ for both training groups.
3.2.2 Sprinting tests
Two way-ANOVA showed no differences between groups. Only 10-m sprint X time had a trend to a significant small effect of time ES=0,08 F(2,38)=3.03 P= 0,07. None of the groups gained a significant decrease in sprinting times from pre to post intervention.
4 Discussion
The purpose of this study was to evaluate the effect of flywheel resistance on neuromuscular performance in physically active youth in the age of 12-14. The study shows that a six-week intervention of flywheel resistance squats, performed as four sets with six repetitions, was insufficient to induce a significant mean effect on any of the measured neuromuscular performance tests in this study’s cohort. Traditional strength training performed as a barbell back squat, was also insufficient to affect the sprinting and vertical jumping ability in the same population. In all, there were almost no interaction effect for any of the variables used in the study. The only significant effect on neuromuscular performance of this intervention was an increase in Squat jump and Countermovement jump for the control group. Both training groups gained significant increases in body mass, while no significant changes occurred in body mass for the control group.
A general and important finding was that no musculoskeletal injuries were reported during the study, which indicates that flywheel training can be used as a resistance training method in youth. Though, whether it’s as good as traditional strength training remains unclear. Previous research shows that appropriately designed and supervised resistance training is safe to perform for youth athletes and this study adds another potential resistance training method to
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that list. (Zwolski, Quatman-Yates & Paterno 2017; Faigenbaum & Myer 2010; Lloyd et al. 2014a). Flywheel training enables the possibility for an experienced athlete to achieve high muscle activation in both the concentric as in the eccentric phase of the movement. This, due to the accommodated resistance that is a product of the intensity performed by the exerciser himself. It seems that the ability to activate a muscle differ between experience and non-experienced (Seger & Thorstensson 2000; Duchateau, Semmler, & Enoka 2006). Flywheel resistance training would in that case probably favor more experienced athletes since they are familiar with producing force in a proficient technical manner. The participants of this study were unfamiliar with resistance training before the intervention and sound technique was the main focus during training. High movement speed when performing flywheel training seems to induce great increases in vertical displacement (Naczk et al. 2016). But the ability to perform a flywheel squat with high speed was not prioritized during this study. A longer training intervention of more than 10 weeks would probably induce larger effects in youths due to greater technical skill level.
Flywheel resistance training with a similar study design as this study has produced performance enhancements functional neuromuscular performance in adult participants (Núñez et al. 2018; Sabido et al. 2017; Maroto-Izquierdo et al. 2017a; Naczk et al. 2016; De Hoyo et al. 2015; Fernandez-Gonzalo et al. 2014). Similar sample sizes (n = 10-15) have induced performance gains for both flywheel studies in adults (Núñez et al. 2018; Sabido et al. 2017; Maroto-Izquierdo et al. 2017a) but also for strength training in youth pre-PHV (Lloyd et al. 2016b). There are some methodological differences for some of the mentioned studies in comparison with study as, a greater training load consisting of 15 sessions
(Fernandez-Gonzalo et al. 2014; Naczk et al. 2016). Some have also used team sport athletes that are jumping in a daily basis such as soccer players (De Hoyo et al. 2015), handball players (Maroto-Izquierdo et al. 2017a; Sabido et al. 2017) and mixed team sport players (Núñez et al. 2018). Youth athletes might, therefore, need plyometric training or experience with jumping mechanics as Rumpf et al. (2012) reported. The short period of 6-weeks and 12 training sessions was probably the main reasons to the non-significant change in functional neuromuscular performance from flywheel training for this pre-PHV cohort. The use of experienced youth athletes, additional training sessions and higher movement speed in flywheel training should be studied in future research.
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Despite no significant changes in the mean, there are slight tendencies in the descriptive statistics that both flywheel training and the traditional strength training had a small effect on 10- acceleration and squat jump (table 3). The training program of this study aimed at
increasing the maximal strength in the squat movement. Although the 1RM was not tested in this study, it’s more than likely that the 1RM increased in both training groups as strength training has been shown to induce gains in maximal strength for youth athletes. There appears to exist a strong correlation between maximal strength, sprinting and jumping performance (Wisløff et al. 2004). The 1RM in a barbell half-squat has a reported strong correlation with 10-m acceleration (r=0,94) and jumping height (r=0,78), but in elite adult soccer players (Wisløff et al. 2004). Squat jump and 10-m sprint are also in similarity with the results of a Lloyd et al. (2016b) that assessed the effect of plyometric-, resistance-, and combined training in youth pre-PHV. The traditional strength training program of Lloyd et al. (2016b) consisted of four exercises with 3 sets of 10 repetitions, where training volume might’ve been the reason to the superior effect in squat jump in comparison to this study’s results. The groups that only performed plyometrics or a combined program of plyometrics and traditional strength assessed superior results in comparison to traditional strength only. The absence of plyometric exercises in this study might be a reason to the non-significant change in
neuromuscular performance. This assumption is supported by the results of the meta-analysis by Rumpf et al. (2012) in which they conclude that plyometric training is the most effective training method to improve sprint times in pre-PHV participants. To explain a potential reduction in sprint times after an intervention one could measure separate variables such as stride length, step frequency and horizontal force to understand what increased. Though, these variables would only be interesting to measure in adults if an actual decrease in sprint time occurred. But as discussed by Rumpf et al. (2012) it might be necessary to measure other performance variables such as stride length, stride frequency and horizontal force when assessing the effect of resistance training on sprint times in youth pre-PHV.
Both training groups in this current study increased in body mass significantly from pre- to post-test assessments, but the control group did not. This might’ve affected the performance of both training groups since increased body mass during puberty has been a negative
influence on sprint times (Meyers et al. 2017). Resistance training for might be the best tool in overcoming negatively influence of increased body mass due to normal growth and
17
and adults has not been investigated after physical training due to the ethical problem of performing invasive measurements of youth muscle. However, short periods of flywheel resistance training have been shown to induce increases in muscle mass (Sanchez & de Villareal 2017). Though, it is unlikely that the increase in body mass of the training groups is due to a gain in muscle. The early-phase adaptions are normally improved neural strength, while morphological and architectural changes require longer time to occur (Seynnes, de Boer & Narici 2007). Further studies on youths after resistance training and the eventual change in body mass with ethically applicable invasive methods would be interesting. Boys and girls in the chronological age of 12-14 are most certainly at the beginning of or mid-PHV. Peak height velocity has been reported to predict the correct biological age with a standard
variation of one year plus or minus (Mirwald et al. 2002). The predicted status in PHV of this current study’s participants was circa 0,8 years to PHV in all groups. Participants could therefore potentially be further or farther in their biological maturation. Further maturational status could potentially be one of the explanations to the increased body mass in both training groups due to the onset of increased hormonal concentrations that occurs during puberty (Lloyd et al. 2014a).
Previous research on resistance training in youths have primarily been studied on boys which makes this study one of the few where the number of female participants exceeded the male number of participants. Due to the limited amount of research in sex-specific effects of resistance training in youth athletes, and especially in respect to biological maturation (Lesinski, Prieske & Granacher 2016), it is difficult to analyze the effect of this current
study’s large amount of youth female participants. Preliminary results from research in female adolescents suggests that girls tend to be as responsive or even more to resistance training in comparison to boys (Lesinski, Prieske & Granacher 2016). Flywheel training seems to induce comparable gains in strength, power and muscle mass in both adult men and women
(Fernandez-Gonzalo et al. 2014). This study will not directly contribute to the current research on sex-specific resistance training effects due to that the participants in this study were block-randomized by peak height velocity status. The sample size is too small to draw any conclusions of the influence in gender for the outcome of this study. It’s also hard to draw any conclusions out of the preliminary findings for female adolescents of Lesinski, Prieske and Granacher (2016) since the mean age in predicted peak height velocity for this cohort
18
categorizes these females into a pre-adolescence group. The sex-specific effect of resistance training in general needs further studying.
One could argue that a light inertia of 0,01 kgm2 would have been more appropriate to start with for youth participants. The main purpose why an initial inertia of 0,025 kgm2 was applied was that the supervising trainer decided that an inertia of 0,01 kgm2 would’ve been harder for the participants to master. This was hypothesized due to the increased movement speed in the concentric phase and the inhibited friction from the device in the eccentric phase which would’ve stopped the flywheel before the strap would fully wind on the shaft of the device. As exercise movement speed increased above the range of 0,5-0,6 m/s, inertia was increased to 0,05 kgm2. Flywheel resistance training with a 0,025 kgm2 has been shown to produce the greatest increases in concentric peak power by Sabido, Hernández-Davó and Pereyra-Gerber (2018). However, the purpose of the current study’s design was to stimulate the maximal force spectra of the force-velocity curve. As the speed of the concentric
movement phase increases the muscle’s ability to produce high force decreases (Seger & Thortensson 2000), we decided to increase the exercise inertia when movement speed and subjectively technical proficiency was performed.
One of the perks that trainers reported with flywheel training in comparison with barbell strength training, was that they perceived it to be easier to assist the participant throughout the entire movement. If the supervising trainer perceived any risk for injury they could physically guide the participant towards a sounder movement or stop the movement by stopping the flywheel manually. Trainers also experienced that participants in the flywheel group
performed a subjectively better squat movement and were able to perform the exercise with higher exercise intensity after a shorter amount of training in comparison with the traditional strength training group. Participants in the traditional resistance training group had a harder time performing a deep squat movement with proficient technical execution. Another perk that was reported by the supervising trainers was that training on the flywheel device was more time efficient than for the traditional strength training group. As the participants in the traditional strength training group fluctuated in technical skill and strength ability the trainers had to spend more time adjusting the load on the barbell. In comparison to the flywheel participants that only had to hand over the harness to the next person and complete a workout with the same amount inertia, still in an individualized way since the performed effort from
19
the participant affects the training intensity. Flywheel training may, therefore, provide a superior time efficient resistance training tool for trainers in an applied setting.
To summarize, further studies on youths performing flywheel resistance training are needed to examine the effects of additional training volume through more training sessions, more resistance training experienced youth, the implementation of plyometric and larger sample sizes. It would also be interesting to implement other methods for assessing the effect of flywheel resistance training on youth performance such as 1RM-testing.
5 Conclusion
This study indicates that flywheel training can be used as a loading option for resistance training on youth athletes without inducing injuries. Flywheel resistance training resulted in a small but non-significant increase from pre to post test in squat jump and 10-m sprint.
Interestingly only the control group increased their performance in Squat Jump and 10-m sprint from pre to post. Further studies of flywheel resistance training in youth athletes with longer training periods, additional training sessions and higher concentric movement speed are needed.
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Appendix 1 - Source and literature search
Purpose and question formulation:
The purpose of this study is to investigate the effect of flywheel training in comparison with traditional strength training on functional neuromuscular performance tests.
What is the effect of flywheel resistance training on 10 m-acceleration in youth athletes?
What is the effect of flywheel resistance training on 20 m-speed in youth athletes? What is the effect of flywheel resistance training on 30 m-acceleration in youth
athletes?
What is the effect of flywheel resistance training on countermovement jump ability in youth athletes?
What is the effect of flywheel resistance training on squat jump ability in youth athletes?
Which words have you been using during your literature search?
Flywheel, resistance training, youth resistance training, eccentric training, reduced injury risk,
Where have you search?
PubMed, Google Scholar,
Which of the literature searches gave relevant results?
PubMed: flywheel resistance training
Google scholar: youth resistance training Google scholar: youth neuromuscular testing
Comments
Many reviews or meta-analyses have given references to original articles that I’ve used in this thesis.
Appendix 2
Information till vårdnadshavare för forskningspersoner
Projekttitel:
Effekten av svänghjulsträning på neuromuskulär prestationsförmåga hos barn och ungdomar
Ansvariga:
Forskningshuvudman: Gymnastik- och idrottshögskolan (GIH).
Kontaktperson: Niklas Westblad, 073 06 073 06, niklas.westblad@student.gih.se
Kontaktperson: Henri Petré, 073 9087911 , henrik.petre@student.gih.se
Försöksledare: Niklas Psilander, 0707 759495, niklas.psilander@gih.se
Bihandledare: Glenn Björklund, 070 3370160, glenn.bjorklund@miun.se
Personuppgiftsansvarig: Gymnastik & Idrottshögskolan
Plats för undersökningen: Sollentuna Friidrottshall, Strandvägen 65, 191 35 Sollentuna.
Bakgrund och syfte
Tiden före och i början av puberteten kan vara ett unikt tillfälle för att utveckla muskelstyrka och grundläggande motoriska rörelsemönster som finns kvar för resten av livet. Den
internationella rekommendationen av världsledande forskare inom fältet för barn- och ungdomsträning är att barn skall börja med åldersanpassad styrketräning vid 6–7 års ålder. Det finns idag god evidens för att individanpassad styrketräning är en av det viktigaste träningsformerna barn bör bedriva för att stärka benmineraldensiteten, öka välbefinnandet, öka prestationsförmågan och reducera skador relaterade tills idrottslig aktivitet.
En träningsmetod som visat sig vara lika effektiv som traditionell styrketräning med fria vikter är svänghjulsträning. Träning med svänghjul sker med en maskin där ett eller flera svänghjul monteras på en axel som är fäst till ett band. En knäböj med sele kan förkorta den tekniska inlärningsperioden i jämförelse med en knäböj med skivstång genom att fördela belastningen utefter rörelseutförandet så den yttre hävstången mot ländryggen förblir kort oavsett utförande. Det finns idag till vår vetskap inga vetenskapliga publicerade artiklar på effekten av svänghjulsträning på barn eller ungdomar i tidig pubertet.
Vi tror att vi kan addera belastning tidigare och därmed på ett tidseffektivt sätt utveckla den neuromuskulära prestationsförmågan i vertikal och horisontell riktning i större förmåga med svänghjulsträning än med traditionell styrketräning med fria vikter.
Metod
Under totalt sex veckor skall idrottande ungdomar före och i tidig pubertet utföra
styrketräning under kontrollerade former. En interventionsgrupp skall utföra knäböj i en svänghjulsmaskin och en annan interventionsgrupp skall utföra övningen knäböj med
skivstång. En kontrollgrupp fortsätter att bedriva den vanliga sportsspecifika träningen under interventionen. Före, efter 3 veckor och efter hela interventionstiden kommer ungdomarna genomföra neuromuskulära tester i form av ett maximalt isometrisk styrketest, hopptester och sprinttester. Detta för att undersöka effekten av svänghjulsträning på barn och ungdomar.
Kunskapsvinster
Det finns idag till vår vetskap inga vetenskapliga publicerade artiklar på effekten av svänghjulsträning på barn eller ungdomar i tidig pubertet. Vi har som hypotes att
svänghjulsträning är mindre tidskrävande vid inlärning av tekniken än skivstångsträning och att vi således kan belasta övningen tidigare med fullgott tekniskt utförande. Detta kan göra att vi kan möta de internationella rekommendationerna kring 2-3 muskelstärkande pass per vecka på ett mer tidseffektivt sätt.
Hur går studien till?
1. Första steget innebär att ni som vårdnadshavare informeras och tillfrågas om erat barn, kan tänkas delta i denna studien. Därefter tillfrågas forskningspersonerna själva, i detta fall erat barn om deltagande i studien. Vi informerar er via ett informationsmöte samt skriftlig information.
2. Andra steget är att båda vårdnadshavarna ger sitt skriftliga samtycke om frivilligt deltagande i studien. Ni får även fylla i en hälsoenkät.
3. Därefter fortsätter studien med att väga och mäta alla deltagare för att uppskatta den biologiska åldern. Denna data kommer att användas för att matcha tre
forskningspersoner med varandra. För denna uträkning behöver vi allas föräldrars längd. Om en förälder inte finns tillgänglig, vänligen uppskatta.
4. Samtliga forskningspersoner kommer sedan att slumpmässigt fördelas i tre
träningsgrupper: en svänghjulsträningsgrupp, en traditionell styrketräningsgrupp (fria vikter) och en kontrollgrupp som inte tränar styrketräning. Innan starten av denna studie ska alla forskningspersoner ha genomfört
till dess att ansvarig testledare anser att individens teknik är fullgod. Ingen kommer att få delta utan att fullvärdig teknik uppvisas.
5. Efter genomförda familjäriseringspass kommer deltagarna att genomföra hopp- och sprinttester samt ett test av den maximala viljemässiga isometriska styrkan. Alla test genomförs vid samma testtillfälle.
6. Efter genomförda förtester kommer interventionsgrupperna (de grupperna som tränar) att under sex veckor genomföra sin intervention i anslutning till två sportspecifika pass. Beräknad tidsåtgång beräknas till 20 – 30 minuter med uppvärmning inkluderad. Kontrollgruppen kommer att fortsätta träna den sportspecifika träningen. Syftet med kontrollgruppen är att kunna visa om den adderade styrketräningen ger en effekt eller inte på den viljemässiga maximala styrkan samt hopp- och sprintförmåga.
7. Efter avslutad träningsperiod kommer återtester att genomföras med samma testförfarande som vid förtesterna.
Möjliga följder och risker med att delta i denna studie
Ingen av de testerna som forskningspersonerna skall genomföra brukar upplevas som mentalt eller fysiskt obehagliga. Med en väl genomförd uppvärmning föreligger det en låg skaderisk i hopp och sprinttesterna då deltagarna endast kommer att arbeta med den egna kroppsvikten som belastning. Det maximala viljemässiga styrketestet är ett test med väldigt låg risk som sker genom att barnet får pressa med fötterna mot två kraftplattor mot ett statiskt motstånd. Det sker alltså ingen rörelse utan kan enklast beskriva som ett ”Svärdet i stenen test”. Som med all form av träning finns det en risk för skada men alla föreskrifter för att motverka dessa kommer att iakttas.
Det finns idag inga studier som visar på att styrketräning för barn- eller ungdomar skall vara skadligt om träningen genomförs under uppsyn av professionella tränare och med en anpassad dos. Testledaren (TL) och ansvarig för studien har mycket god erfarenhet av styrketräning för barn och ungdomar. TL är utbildad i svänghjulsträning och arbetar dagligen med
styrketräning för barn och ungdomar. Belastningen för varje deltagare kommer att vara individanpassad och teknik prioriteras alltid före belastning.
Hantering av personuppgifter och sekretess
Vi kommer att samla in data i form av forskningspersonernas namn och födelsedatum, tidigare träningshistorik och eventuell skadehistorik. Hälsoenkäten är enbart till för att forskarna ska kunna säkerställa att forskningspersonen är frisk och kan delta i studien. Data från hälsoenkäten kommer inte att användas till analys eller publicering. Hälsoenkäten kommer att förstöras när insamlingen av data är färdigställd.
Syftet med undersökningsresultaten av de neuromuskulära testerna är att studera effekterna av träningsinterventionen. Undersökningsresultaten kommer att användas för att på gruppnivå
publicera resultaten i vetenskaplig tidskrift samt masteruppsats. Vid publicering av forskningsdata kommer dessa inte kunna kopplas till forskningspersoner som individ. Undersökningsresultaten kommer löpande kodas vilket innebär att inga resultat kan kopplas till en individ utan en kodnyckel som är inlåst och endast finns tillgänglig för ansvariga forskare. Undersökningsresultaten kommer inte att användas för att besvara andra frågeställningarna utanför denna studie. Ni har rätt att ta del av erat barns resultat och få rättelse av eventuella felaktiga personuppgifter (personuppgiftsansvarig se ovan).
Undersökningsresultaten sparas till dess att analys är färdigställd. Därefter kommer samtliga resultat förstöras. Era personuppgifter kommer att behandlas enligt gällande
dataskyddslagstiftning och Gymnastik- och Idrottshögskolan i Stockholm är huvudansvarig för behandlingen av testdata.
Resultatåtergivning
Efter fullföljt deltagande i studien kan vi skicka resultaten till er vårdnadshavare. Dock kan vi inte garantera anonymitet vid mailåterkoppling då vi behöver använda kodnyckeln för att knyta resultatet till respektive person.
Försäkring och ersättning
Under denna studie är ditt barn försäkrat genom GIH. Ingen ersättning utbetalas för fullföljt deltagande.
Deltagandet är frivilligt
Ditt barns deltagande är frivilligt och hen har rätt att när som helst avsluta sin medverkan i denna studie. Vid en eventuell avslutad medverkan behövs inget skäl anges. Om ditt barn vill avsluta sitt deltagande kontaktar ni ansvarig föra studien.
Vid eventuella frågor kontakta ansvariga för studien.
För mer information kontakta
Försöksledare: Niklas Psilander, 0707 759495, niklas.psilander@gih.se
Samtycke till att delta i studien
Projekt: Effekten av svänghjulsträning på neuromuskulär prestationsförmåga hos barn och ungdomar
Vi har muntligen informerats och har fått tillfälle att ställa frågor. Vi har tagit del av
ovanstående skriftliga information och samtycker till deltagande i studien. Vi är medvetna om att vårat barns deltagande är helt frivilligt och att barnet när som helst och utan närmare förklaringar kan avbryta sitt deltagande.
……… …….……….. datum Vårdnadshavare 1: namnteckning och namnförtydligande
……… …….……….. datum Vårdnadshavare 2: namnteckning och namnförtydligande
……… …….……….. datum Ansvarig: namnteckning och namnförtydligande
Kroppslängd för biologisk åldersuppskattning. Längd – Vårdnadshavare ________________ Längd – Vårdnadshavare ________________
